GNSS & Positioning

GNSS is the general term for satellite positioning constellations, of which GPS is the best known. A receiver measures the travel time of signals from several satellites and solves for its own position. The method depends entirely on receiving microwave signals from orbit, and that is exactly what stops at the sea surface. Seawater is electrically conductive and attenuates radio frequencies severely; at GNSS frequencies a receiver loses lock within centimetres of submersion. There is no underwater GPS and there is not going to be one.

What replaces it is acoustic, because sound travels well in water. Acoustic positioning measures the time of flight of a pulse between transducers and converts it to range using an assumed speed of sound, which is near 1,500 metres per second but varies with temperature, salinity, and pressure. Sound speed error is the dominant error source in most installations, which is why survey crews take velocity profiles through the water column and apply them to the solution.

Architectures differ by baseline, meaning the separation between the reference points used to fix a position. Long baseline uses transponders deployed on the seabed across a wide area and surveyed in beforehand; it gives high accuracy largely independent of water depth, at the cost of deployment and recovery time. Short baseline uses transducers mounted at separated points on a vessel hull. Ultra-short baseline puts a compact array in a single head, taking range from travel time and bearing from the phase difference across closely spaced elements, which makes it quick to mobilise but sensitive to how well the vessel attitude and heading sensors are calibrated against it.

Every subsea position ultimately references back to a satellite fix at the surface, so the error budget stacks: satellite solution, vessel motion measured by an inertial unit, alignment offsets between sensors, and the acoustic range itself. Inertial navigation aided by a Doppler velocity log fills the gaps between acoustic updates, and it is how untethered vehicles hold a position between fixes.